1. Field of the Invention
The present invention relates to a laser element in which a laser bar being made of nitride-based semiconductor materials and containing a plurality of light-emission points formed on a single substrate is fixed to a heat sink. The present invention also relates to a process for producing the above laser element. The present invention further relates to a laser module containing the above laser element.
2. Description of the Related Art
The following documents (1) and (2) disclose information related to the present invention.
(1) U.S. patent Laid-Open No. 20020090172
(2) Roland Dichel (ed.), “High-Power Diode Lasers: Fundamentals, Technology, Applications, With Contributions by Numerous Experts,” Springer-Verlag, 2000
In order to increase laser output power, laser modules which comprise at least one laser-light source for emitting a plurality of laser beams, an optical fiber, and an optical system have been proposed, for example, as disclosed in document (1). The optical system comprises a plurality of collimator lenses and a condensing lens for optically multiplexing the plurality of laser beams emitted from the at least one laser-light source. The at least one laser-light source can be realized by arranging on a heat sink a plurality of laser diodes each having a single light-emission point, or using a semiconductor laser bar in which semiconductor layers are formed on a single substrate so as to realize a plurality of light-emission points.
However, in semiconductor laser bars in each of which a plurality of light-emission points are formed by crystal growth on a single substrate, a so-called “smile”, which is a curvature of the array of the plurality of light-emission points, is produced, and a portion of a plurality of laser beams are displaced upward or downward from positions at which the portion of the plurality of laser beams should be arranged, as explained in, for example, the aforementioned document (1). Conventionally, each of the above semiconductor laser bar is bonded onto a heat sink having a flat fixation surface while correcting the curvature by pressing a columnar collet to the center or its vicinity of the semiconductor laser bar in the direction in which the plurality of light-emission points are arrayed.
For example, in the case where the magnification power of an optical system in a laser module as mentioned before is about 10, and the light-emission-point displacement (defined later) in a semiconductor laser bar is about 2 micrometers, the light convergence point is displaced by a maximum of about 20 micrometers (=2 micrometers×10) at a light-entrance end of an optical fiber. That is, the light convergence spot is displaced by a maximum of about 20 micrometers from the center of an optical fiber. In the case of the typical optical fiber having a radius of 25 micrometers, the above displacement causes significant decrease in the coupling efficiency. The light-emission-point displacement is defined as the difference between the maximum and the minimum of distances from a fixation surface of a heat sink to the centers of light-emission points in a place perpendicular to the optical axis of lenses.
On the other hand, conventionally, indium, which has low rigidity (i.e., is soft), is used for bonding laser bars made of GaAs-based semiconductor materials to heat sinks made of copper. Since the thermal expansion coefficients of the copper heat sinks and the laser bars made of GaAs-based semiconductor materials are greatly different, heat generated by operation of the semiconductor laser bar causes internal strains in the semiconductor laser bars. However, when indium is used for the bonding, the internal strains can be relieved.
The nitride-based semiconductor laser bars which are formed by growing GaN-based compound semiconductors on a sapphire or GaN substrate so as to realize a plurality of light-emission points generally have higher rigidity than the GaAs-based compound semiconductor laser bars. Therefore, it is impossible to deform each nitride-based semiconductor laser bar so as to have a shape equivalent to the surface of a heat sink, even when an attempt is made to bond the nitride-based semiconductor laser bar to the heat sink by pressing, with a collet as mentioned before, only the center or its vicinity of the semiconductor laser bar in the direction in which the plurality of light-emission points are arrayed. In addition, when indium is used as a soft brazing material, it is impossible to fix the nitride-based semiconductor laser bar to the heat sink so that the nitride-based semiconductor laser bar has a shape equivalent to the surface shape of a heat sink, because the hardness of the nitride-based semiconductor laser bar is high.
Further, in the case where nitride-based semiconductor laser bars are used in laser modules as mentioned before, the aforementioned curvature decreases the coupling efficiency to the optical fiber. Furthermore, each nitride-based semiconductor laser bar tends to revert toward a shape having the original curvature. Therefore, due to heat generation during operation and other reasons, the coupling efficiency decreases as time elapses.